The present invention generally relates to a xerographic reproduction machine and, more particularly, to a control system for monitoring toner consumption in a xerographic plotter and for adjusting toner concentration levels accordingly.
In xerographic type reproduction machines, latent electrostatic images of a document being copied or printed are generated on a charged surface of a moving recording member, such as a drum or belt photoreceptor, through exposure of the document being copied or in accordance with an electronic image signal input. Following exposure, the latent electrostic images on the recording member are developed at a developing station, which, in typical present day practice, comprises one or more magnetic brushes for bringing a developer, usually a mixture of carrier beads and toner, into developing relation with the recording member and the image thereon. Following this, the developed image is transferred at a transfer station to a copy substrate material such as a sheet of paper. After transfer, the copy substrate material is fixed, as by fusing, to provide a permanent plot or reproduction.
In the course of developing images as described above, the toner portion of the developer mixture is depleted and, to maintain the necessary proportion of toner to carrier, fresh toner must be added from time to time.
Various types of toner re-supply systems are known in the prior art, as for example, the canister or cartridge type disclosed in (Del Vecchio et al) U.S. Pat. No. 3,337,072. In Del Vecchio, a toner supply canister consisting of relatively rotatable inner and outer concentric tubes, each with a toner dispensing opening, are used. The supply of fresh toner is held in the inner tube, and by rotating the inner tube relative to the outer tube, the toner dispensing openings in each are brought in alignment. Another system is shown in (Eichorn) U.S. Pat. No. 3,339,807. There, the toner supply canister, once mounted, rotates to bring the toner dispensing holes opposite a series of openings in a stationary grid. Preparatory to this, a tear away strip, which seals the holes during shipment, is first removed. In another prior art system shown by (Navone) U.S. Pat. No. 4,089,601, a toner canister housing is installed in the machine following which the canister is turned to communicate the toner dispensing openings within a developer sump. In (Manno) U.S. Pat. No. 4,688,926, a rotatable toner supply cartridge has a row of toner discharge ports with a toner rejecting rod with flats opposite each port. The rod is periodically rotated coincident with cartridge rotation to provide a dispensing of a controlled amount of toner into a developer housing beneath the dispenser.
These prior art rotating dispensing systems have a common characteristic in that the cartridge is periodically rotated at fixed time intervals. This characteristic has two inherent defects which have heretofore been tolerated because of the nature of the typical copying job (e.g., copying of letter or legal size documents). The first defect is that the toner dispense cycle is constant irrespective of the nature of the image being developed. As an example, a first series of documents which are to be copied may consist of white background with very little line information. A second series may consist of a black background with white information. Development of the first series of documents result in very little toner depletion. Development of the second series causes substantially greater depletion. The toner dispense timing cycle remains the same for both sets of circumstances. Thus, the prior art dispensing systems do not recognize the different informational content of documents being copied and do not adjust a dispensing rate accordingly. The second factor is that the amount of toner dispensed with each cycle becomes progressively less. This is due to the fact that when the cartridge is relatively full, the weight of the toner increases the toner dispense through the dispense openings or ports. As the cartridge begins to empty, the weight and the amount of toner gravity fed through the ports during each dispense cycle becomes progressively less. This gradual decrease in toner release results in some degradation in developed image quality. Various toner density sensing schemes are known in the art to detect this toner density decrease, either by using optical sensors in the discharge developer housing, or by developing a test image density and sensing the test image with a densitometer whose output activates the toner dispensing mechanism.
These prior art toner dispensing devices, while tolerable in conventional copiers, present serious problems with xerographic plotters such as the Xerox 8836 Plotter which receives electronic image inputs from, for example, a CAD/CAM unit, the image input then being converted into light beams which expose a photoreceptor in the image pattern. This type of plotter is capable of receiving CAD/CAM inputs representing documents up to 15 feet in length or more. In developing documents of this length (following the exposure step), it becomes increasingly important to maintain the desired development density during a time interval which is substantially longer than the typical copying job. The density of document information and the gradual density degradation in a toner dispensing supply constitute a serious problem in maintaining optimum output copy appearance. According to a first aspect of the present invention, the total number of dispense cycles the toner cartridge undergoes before being depleted is counted and stored. Two or more regions of this cycle are identified as having an average dispense amount which differ due to the uneven dispense rate described above. Means are provided to count each dispense rotation and to identify which cycle regions the dispenser is operating in. The video image data input is monitored and means are provided to rotate the dispenser only when a specified number of data bits have been "previewed". The dispenser rotation is increased as the dispenser cycle enters regions of progressively greater toner depletion. Thus, a main object of the invention is to dispense toner on a basis which reflects both the informational content of the image to be developed and the number of dispense cycles already accomplished. This results in the density of the developed image being constant and being maintained at a optimum value through a relatively long cycle without intercopy gaps where sensors are usually viewed.
According to a second aspect of the invention, additional toner and dispense cycles are initiated based on a detection of a length of the document being processed. The additional dispense cycles have been found necessary to replenish toner which had been attracted to the white background areas of the processed image, e.g. the attraction of "wrong polarity" toner. This white background depletion phenomena is ordinarily not a problem with conventional copiers copying conventional size documents but, when printing lengthy documents such as low area coverage engineering drawings, the toner depletion becomes a factor and must be compensated for.
More particularly, the invention is directed towards a development system for developing an electrostatic latent image on a photoreceptor, the development system including, in combination, a developer housing adjacent said photoreceptor with means in said housing to bring developer from said housing into developing relation with said photoreceptor surface to develop said latent electrostatic image, a toner cartridge adapted to periodically rotate and dispense a supply of toner into said developer housing, and toner dispensing control means adapted to determine the total number of dispense cycles the cartridge has undergone and to weight the periodicity of said toner dispensing as a factor of the total number of dispense cycles.